This invention relates to improvements in high fired basic, direct bonded, magnesia-chrome ore, also referred to as magnesite-chrome, refractory shapes and the method of manufacture of such improved shapes. More specifically, refractories made in accordance with this invention are provided which yield significantly improved service life in severe wear zones of industrial furnaces as compared with the performance of a variety of refractory compositions and products commercially available today.
Basic, direct bonded, magnesite-chrome refractory brick represent an important, if not the most important, class of refractory employed as a furnace lining.
The state of the art of conventional direct bonded refractories, presently so relied upon by industry, is well developed in the United States. The introduction of direct bonded refractories early in the 1960's was made possible by the availability of relatively high purity raw materials, especially washed or concentrated chrome ores wherein the SiO.sub.2 content was reduced from 4 to 6% down to below 2%, and as low as 1% for ore of African origin known as Transvaal concentrates. Similarly, beneficiated chrome ore with SiO.sub.2 content of 1.5 to 3% became available from the Philippines.
These ores in combination with synthetic periclase or dead burned high purity magnesite containing less than 1.5%, preferably less than 1%, SiO.sub.2, could be processed into brick shapes in the conventional manner and fired at temperatures higher than 1650.degree. C. (3000.degree. F.) without excessive slumping or sticking. As is well described in the literature, the direct bond so developed is the result of high temperature interactions between the chromite and magnesia, involving solid state reactions, solution-precipitation reactions and redistribution of silicates which were present in the raw materials as accessory mineral phases. Most desirably, the periclase crystals are sintered directly to chrome ore, periclase is bonded to periclase, and secondary spinels bond periclase crystals. Additionally, some silicate bonding may co-exist.
In the manufacture of direct bonded brick, size graded magnesia and chrome ore are mixed with temporary binders and pressed at pressures exceeding 5000 psi, for example as high as 16,000 psi, dried, and fired at temperatures above 1650.degree. C. Materials, sizing, and processing are adequately described in U.S. Pat. No. 3,180,744.
Refractories important in the 1950's, such as silica brick and chemically bonded basic brick, have largely been replaced by high fired 50, 60 and 70% MgO direct bonded brick in open hearth roofs, walls and uptakes, electric arc furnace walls, and copper converter linings. The newer special processing units such as vacuum degassing and Argon-Oxygen Decarburization (AOD) vessels are currently lined extensively with direct bonded brick. The 60% MgO class is dominant because it represents a desirable economic balance between costs, chemical resistance and physical properties.
Service life or productivity has been generally improved in many of the furnaces employing direct bonded brick thereby leading to new efforts to balance furnace wear by zoning with improved products. Many materials have been tried, such as fused cast basic, rebonded fused grain brick, and direct bonded brick of higher MgO class, but in most cases each was found to have inherent disadvantages.
The more expensive fusion cast basic brick, while similar in chemical composition to conventional direct bonded basic, is extremely dense and essentially free of micropores. Although this product is highly resistant to slag erosion, it often fails due to spalling or cracking and bulk loss initiated by thermal shock stresses. These features detract from its use in modern high production open hearth roof center sections and backwalls as well as AOD tuyere lines and the like.
Rebonded fused grain brick products are even more expensive than the premium priced fused cast shapes for most conventional applications. While somewhat less slag resistant than fusion cast brick, these refractories, where not prone to spalling are often uneconomical at nearly two times the price of conventional direct bonded brick.
Another approach known in the refractories art to produce basic brick with improved service life is to prereact or sinter together the periclase of MgO source, which can be magnesium hydroxide, magnesium carbonate or caustic magnesia, with ground, sized chrome ore at high temperature, e.g., above 1700.degree. C. (3100.degree. F.), to form direct solid-solid bonding in grains prior to sizing the aggregate for brick forming. In most cases, the shape is fired at temperatures above 1600.degree. C. (2910.degree. F.). The method of manufacture and properties of basic brick formed from prereacted grains are outlined in Austrian Pat. No. 189,113 and corresponding U.S. Pat. No. 3,429,723.
Unfortunately, it is commercially difficult to produce brick from prereacted grains due to the fact that it is often undesirable to contaminate a periclase grain-producing plant by introducing chrome ore. In a conventional brick plant, the use of a prereacted grain often means an additional, costly processing line to avoid chrome contamination of the periclase grinding and batching system. In any event, while such prereacted brick possess desirable strength and slag resistance, they are less resistant to thermal shock than conventional direct bonded brick and therefore have not proven to be a suitable material for certain severe wear areas, especially where spalling is a factor.
It has even been proposed to improve the strength and further lower the porosity of prereacted magnesite-chrome grain refractory products by adding from 3 to 6% chromic oxide to the refractory batch prior to forming and firing. The method of manufacture and improved properties of such prereacted basic refractory brick is described in U.S. Pat. No. 3,594,199. While strength is markedly increased and porosity further reduced by this process, brick made therefrom remain less resistant to spalling or thermal shock than conventional direct bonded brick. Moreover, with the prefiring of nearly all the raw materials, and refiring in brick shape form, plus the addition of pure chromic oxide, the selling price required to cover costs and reasonable profit is necessarily high, as high as rebonded fused grain basic brick. Such brick incorporating chromic oxide has previously been suitable for use in only selected limited applications, where furnace shutdowns or severe thermal shock is absent.
It is further known in the art that chromic oxide added to a refractory batch acts as a pressing aid or lubricant to increase the "as pressed" density and density after firing of many classes of refractories, including magnesia, magnesia-chrome, alumina, and zirconia. A portion of the increased density is due to the substitution of chromic oxide material for ingredients of lower specific gravity. The specific gravity of Cr.sub.2 O.sub.3 is 5.1 to 5.2 while that of periclase is 3.5 to 3.6. In any event, corresponding porosity improvements are cited for such brick in U.S. Pat. No. 3,192,058 which discloses the method of manufacture. However, the use of chromic oxide must be confined to very specific compositional fields in order to realize improved service results commensurate with the increased manufacturing costs incurred with its use. Benefits beyond the effects of higher density and reduced porosity must be achieved in order to make a significant improvement in service life in the lining of an industrial furnace. Therefore, chromic oxide has not been used commercially as extensively as might be inferred from the literature.
A primary object of this invention is to provide an economical, improved high fired direct bonded basic refractory shape combining both increased resistance to slag penetration and resistance to thermal shock, spalling or "slabbing", equal to or better than conventional direct bonded shapes.
An additional object of this invention is to provide a method of manufacture for an improved high fired direct bonded basic refractory shape.
A further object of this invention is to improve the microstructural features of direct bonded basic refractory shapes which in turn improve slag resistance and physical properties.
Another important object of this invention is to provide an improved high fired direct bonded basic refractory brick which gives improved service life in linings of industrial furnaces, and a method of manufacture for such a brick.
These and other objects will become apparent from the specification and claims.